Slebech Park is a private estate on the north side of the eastern Cleddau river. There are plenty of signs saying where you can and can't go, but also some good public footpaths. The remains of the church of St John the Baptist are situated on the river bank, and are accessible. At one end, the font is still in place and a short, very narrow spiral staircase leads part way up the tower. Just offshore is a small island (on the right in the photo) known locally as Dog Island.
Access here is by public footpath. One way is from Blackpool Mill (almost 2 miles), but a shorter route is from a lay-by on the A40 (south side, Grid Ref : SN038150).